1. Technical Field of the Invention
This invention pertains to integrated circuits. In particular, this invention pertains to a frequency dependant impedance device for providing a high impedance point in a circuit without requiring off chip components.
2. Background Art
Two implementations of frequency dependant impedances are outlined below. First, biasing for a PIN diode. PIN diodes can be thought of as light controlled current sources. For differential sensing one or both sides of the PIN diode must be AC coupled to a differential transimpedance amplifier (FIG. 3B). For typical data rates (ATM, FDDI) the lower cutoff needs to be around 200 KHz. In order to allow the use of an on die AC coupling capacitor in the range of about 10 pF a large input impedance is required. Use of resistors (FIG. 2B) is not feasible since the PIN can have DC current approaching 1 mA which would prohibit the use of large resistors with a 5 V supply. Diode biasing (FIG. 2C) is also unacceptable because it would require off-chip capacitors since the diodes have a low impedance for small signals. Inductors (FIG. 2A) also would require off-chip components.
Another scenario outlined below includes biasing an MR (magneto-resistive) head. Typically, one side of the head is connected to a current source while the other side of the head, or common mode voltage, is held at a voltage by an amplifier. Ideally, the amp has a current source output. A common problem with a current source output is that an inverting device has a large gain from its gate/base to its drain/collector due to the high impedance of the current source biasing the MR. This gain cause problems when trying to stabilize the amplifier. "Millering" the output device is not an option since the output impedance will go down as the output device is AC coupled into a diode. Off chip capacitors are typically frowned upon because of packaging constraints.
The present invention is especially helpful when a large impedance is required while biasing a device which needs a variable or DC current to operate.